Preheat the oven to 375°F. Spray a 13 x 9 inch baking dish with cooking spray. Set aside.
In a large bowl, use a pastry blender to cut cold butter into the flour until it resembles coarse crumbs.
Add the buttermilk mixing just until combined. Using floured hands, press the dough onto the bottom of the baking dish.
Sprinkle the sausage crumbles over the biscuit crust.
In a medium mixing bowl, whisk together the eggs, sour cream, milk, grated Parmesan cheese, ground mustard, garlic salt and black pepper.
Stir in the hash browns, 2 cup shredded cheese, and green onion.
Pour the filling evenly over the sausage.
Sprinkle the remaining cheese on top.
Bake for 45-50 minutes or until a knife inserted into the center comes back clean. Rest for 10 minutes, then serve.